PowerMax: Reclaiming space on a linked target device

Snapshots are not full copies. They are "space saving replicas." 
 
When a target device is linked to a snapshot, PowerMax presents the host with a combination of allocation types. Those include:

1. Unchanged source track allocations
2. Not-yet-allocated track allocations
3. Track allocations being protected for multiple snapshots (this snapshot and others). These are "shared" allocations.
4. Track allocations being protected exclusively for this snapshot
5. If the target is then written to by the host, these host-written track allocations are also presented (and consume their own space from the Storage Resource Pool (SRP)).

This presentation of the linked target device provides the host with a fully functional illusion of access to the entire capacity of the target devices. They are equal to the provisioned capacity of the source devices.

When a given snapshot is terminated, only type 4 allocations from that list are freed into the SRP. All remaining tracks remain allocated for the other devices/snapshots still using them.

Freeing the type 5 allocations to the SRP requires a Free -all operation against the target device. The best practice is to do this while the target is no longer linked. 

Note: This operation against a target device does not return any allocations of types 1-4 above, only type 5.

For example:

• • Device 0082 was a target linked to a snapshot for source device 0052, which is a 2 TB device that is 25% utilized (500 GB).
  • Host mounts this device, sees it as a 2 TB device and begins writing to it. An additional 250 GB in written allocations are generated by this host. 
  • The allocations are checked using symsnapvx list -sid xx -dev xxxx -detail -GB, which shows that 80% of the target device's utilized capacity is shared.
  • Now, the total allocations for each type in this scenario looks like this:
    1. Unchanged source track allocations: 250 GB
    2. Not-yet-allocated track allocations: 1500 GB of the capacity, but not utilized/allocated
    3. Shared allocations: 200GB
    4. Exclusive allocations for this snapshot: 50 GB
    5. Host-written track allocations on the target: 250 GB
  • A total of 500 GB of snapshot and target allocations, with an additional 250 GB still owned by the source device
  • Target device 0082 is unmounted, unlinked, and the snapshot is terminated. 
    • Only type 4 is freed, returning 50GB to the SRP.
  • Target device 0082 receives a free -all
    • At this point only type 5 is freed, returning 250 GB to the SRP.
  • The remaining 200GB of shared allocations remain in place because they have other dependencies (other devices or snapshots still need that data).

Deduplication Note: Deduplicated data is not freed by the above processes because other dependencies remain.